Field of the Invention
The present invention relates to an improved process to remove acid forming gases from exhaust gases. Specifically, the process relates to the use of an aqueous alkaline bisulfite/sulfite with a compound to convert NO.sub.x and/or HNO.sub.2 rapidly to environmentally acceptable nitrogen.
The present invention relates to an improved process for the removal of acid gases including NO.sub.x from exhaust gases, particularly to a combined removal of NO.sub.x and SO.sub.2 from flue gas and the like and also to the acquisition of valuable products from the process. (Flue gas usually contains both nitric oxide (NO) and nitric dioxide (NO.sub.2); these oxides of nitrogen are collectively given as NO.sub.x.
Concerns about air pollution caused by acid rain are increasing world wide, and considerable research effort is being expended to provide effective treatment of flue gases and other exhaust gases to remove acid forming components therefrom. However, the present methods have disadvantages which are particularly acute with respect to the removal of NO.sub.x. In addition, the present methods are extremely costly.
Early methods were primarily used to remove pollutants when the concentrations were very high. As time goes by, and larger volumes of gases are generated, tolerable levels of emissions keep getting lower and lower. At this time emissions may be treated to obtain acceptable levels of SO.sub.2 by means of scrubbing processes using aqueous solutions. However, removal of NO.sub.x presents problems, the most serious being sufficient removal and economic considerations. In addition, the economics of using two processes has prompted efforts to utilize wet scrubbing for removal of both NO.sub.x and SO.sub.2 in a single process, and some success has been achieved in this direction. Due to the difficulty in solubilizing NO in aqueous solution, these processes have utilized expensive ingredients and often have provided other products requiring disposal.
Wet processes developed for removal of NO.sub.x have been reported. For example, Patent No. P 32 38 424.6 issued by the Federal Republic of Germany Apr. 19, 1984 to Hoechst AG utilizes red phosphorus in inert oxidizing media to remove NO and NO.sub.2 from flue gas. However, the patent reports the treatment of very high concentrations of NO, typical concentrations being up in the thousands of parts per million, and in Example 7 of the patent where 1000 parts per million were treated, only 40% was removed. In the two part Example 9, the patentee reports 14,000 parts per million were treated in the first step to obtain a 90% removal to 1,300 parts per million; and in the second part about a 65% removal to about 460 parts per million. Such effluent concentrations are not sufficiently low enough, and we have found that red phosphorus is not satisfactory to treat concentrations of 500 parts per million or less.
Current NO.sub.x standards for power plant emissions may be attainable using the selective catalytic reduction (SCR) process which is very expensive. In addition, there is very limited experience with SCR on US coal with high sulfur content and variable ash composition. High SO.sub.2 concentration promotes the formation of ammonium sulfate/bisulfate particulates, which result in the plugging of air heaters of boilers. Ash composition rich in arsenic and alkali could be detrimental to catalysts employed in the SCR system. Other approaches for the reductions to amounts less than 100 ppm are reported in U.S. Pat. No. 4,079,118 entitled Method for Removing Nitrogen Oxides Using Ferricion-EDTA Complex Solutions issued Mar. 14, 1978, and various other wet processes have been developed to provide efficient removal of NO.sub.x. However, these processes generally require either the use of expensive starting materials or create a disposal problem for the products of the processes or both.
Numerous other patents have been issued which disclose wet processes for removal of NO.sub.x such as U.S. Pat. No. 3,984,522; U.S. Pat. No. 4,079,118 and U.S. Pat. No. 4,158,044. In addition, many patents have issued which disclose combined processes for removal of both SO.sub.2 and NO.sub.x. Examples of such patents include U.S. Pat. Nos. 4,126,529 and 4,347,227. Many other systems have been suggested, and the list is too long to include them all. However, there is much room for improvement in providing a practical, efficient removal process for both of such pollutants either individually or together.
As mentioned above, sulfur oxides can be effectively removed by flue gas desulfurization scrubbers. The majority of these scrubbers now in use involve wet limestone processes, which utilize aqueous slurries of limestone to neutralize the sulfurous and/or sulfuric acids produced from the dissolution and subsequent oxidation of flue gas SO.sub.2 in scrubbing liquors. The resulting solid slurries, containing CaSO.sub.3.1/2H.sub.2 O and gypsum (CaSO.sub.4.2H.sub.2 O), can be hauled away for disposal. Such practice is common among power plants located in areas where landfill space is abundant. On the other hand, the more practical solution for power plants situated in densely populated areas is to operate the scrubbers under forced oxidation conditions. Under those circumstances, the major by-product of the scrubbing process is gypsum, which is of some commercial value as a building material.
Further versatility in the processing by flue gas desulfurization scrubbers is obtained by utilizing other alkalis besides limestone or lime. These include soda ash (Na.sub.2 CO.sub.3), nahcolite (NaHCO.sub.3), trona (Na.sub.2 CO.sub.3 3/NaHCO.sub.3), Na2SO.sub.3, NaOH, KOH, K.sub.2 CO.sub.3 /KHCO.sub.3, magnesite (MgCO.sub.3), dolomite (CaCO.sub.3 /MgCO.sub.3), NH.sub.4 OH, and (NH.sub.4).sub.2 CO.sub.3 /NH.sub.4 HCO.sub.3. These materials are more expensive than limestone and are more often used in chemical industries where the volume of waste gas to be treated is small compared to those from power plants, and where the plants are in close proximity to the production sites of those alkalis.
While the wet flue gas desulfurization scrubbers described above are very efficient in the removal of SO.sub.2 from flue gas, they are incapable of removing sufficient NO because of its low solubility in aqueous solution. NO makes up about 95% of the NO.sub.x in most flue or exhaust gases. The installation of a separate scrubber for flue gas denitrification generally requires additional capital investment. Accordingly, approaches to modify existing wet flue gas desulfurization processes for the simultaneous removal of SO.sub.2 and NO.sub.x emissions have been under world wide investigation.
Several methods have been developed to enhance the absorption of NO.sub.x in scrubbing liquors. These include the oxidation of NO to the more soluble NO.sub.2 using oxidants such as O.sub.3, ClO.sub.2, and KMnO.sub.4, as well as the addition of various iron (II) chelates to the scrubbing liquors to bind and activate NO (See, H. I. Faucett, J. D. Maxell and T. A. Burnett, "Technical Assessment of NO.sub.x Removal Process for Utility Application", EPRI AF-568, EPA600/7-77-127 March, 1978). So far, none of these methods has been demonstrated to be cost effective, despite high removal efficiencies of both SO.sub.2 and NO.sub.x.
EXHAUST GAS SO.sub.2 AND NO.sub.x EMISSION REDUCTION--The requirement to control SO.sub.2 and NO.sub.x emissions to the atmosphere has become mandatory. SO.sub.2 is soluble and can be removed from flue gas by an alkaline aqueous solution and/or slurry. SO.sub.2 is neutralized by alkaline reagents to form bisulfite/sulfite ions; sulfate ion is also formed in the presence of O.sub.2. Processes such as limestone, lime, sodium, Dual-alkaline, Wellman-Lord, Union Carbide's Cansolv and Dow Chemical Company's amine systems are based on this principle. However, these processes are not capable of removing NO.sub.x, because most of the NO.sub.x in flue gas is present as NO, which is substantially insoluble in water.
One approach to circumvent this problem is to oxidize NO to the more soluble NO.sub.2 by an oxidant such as O.sub.3, or ClO.sub.2, or by oxidants produced from the reaction of yellow phosphorus with O.sub.2 (P.sub.4 /O.sub.2). Nevertheless, there are two problems associated with this approach:
(1) the slow hydrolysis rate of NO.sub.2, and
(2) the formation of nitrite and nitrate ions. Various nitrogen-sulfur compounds are also formed in scrubbing liquors when SO.sub.2 is simultaneously removed with NO.sub.2 in one absorber.
The first problem results in the requirement of a large liquid/gas (L/G) ratio in the absorber to achieve a desirable NO.sub.x removal efficiency. The second problem results in a mixture of nitrogen-sulfur compounds and the requirement for the subsequent treatment of the scrubbing liquors containing them.
Numerous patents have been issued which disclose wet processes for the removal of NO.sub.x through the oxidation of NO to NO.sub.2 by an oxidant. These include, for example:
Japanese Patent No. 75,01,964 uses ClO.sub.2 to oxidize NO to NO.sub.2, which is then scrubbed with a Na.sub.2 SO.sub.3 solution.
Canadian Patent No. 1,021,922 and West German Patent No. 2,416,980 both use O.sub.3 or ClO.sub.2 to oxidize NO to NO.sub.2 which is then scrubbed with aqueous NaClO.sub.2 to form HNO.sub.3.
West German Patent No. 2,600,034 uses ClO.sub.2 or O.sub.3 to oxidize NO to NO.sub.2, followed by scrubbing NO.sub.2 with an aqueous mixture containing alkali metal sulfite and a catalyst selected from o-aminophenol, trinitrophenol, triethylenediamine, or triaminophenol.
Japanese Patent No. 78,75,187 discloses the use of ClO.sub.2 to oxidize NO to NO.sub.2 which is then scrubbed with an aqueous thiourea solution.
Japanese Patent No. 79,39,368 uses ClO.sub.2 or O.sub.3 to oxidize NO to NO.sub.2, which is then scrubbed with aqueous NaOH solutions.
Japanese Patent No. 79,35,870 uses ClO.sub.2 to oxidize NO to NO.sub.2 which is subsequently irradiated with UV light.
Japanese Patent No. 90,169,012 uses ClO.sub.2 and then an aqueous Na.sub.2 S solution to remove NO.
In addition, many patents have been issued which disclose the combined removal of SO.sub.2 and NO.sub.x. Examples of such patents include, for example:
Japanese Patent No. 74,130,361, which uses an aqueous lime solution to neutralize SO.sub.2, and ClO.sub.2 followed by an aqueous NaClO.sub.2 to oxidize NO to HNO.sub.3.
Japanese Patent No. 75,10,778, which uses a calcium salt solution to neutralize SO.sub.2 and ClO.sub.2 to oxidize NO to NO.sub.2 which is then scrubbed with SO.sub.3.sup.-2 solutions to remove NO.sub.2.
Japanese Patent No. 75,131,851, which uses ClO.sub.2 to oxidize NO to NO.sub.2 which is then scrubbed with aqueous alkali metal bicarbonate and alkali metal sulfites to remove NO.sub.2 and SO.sub.2.
Japanese Patent No. 75,131,853, which uses sodium and calcium salt solutions to neutralize SO.sub.2, and ClO.sub.2 to oxidize NO to NO.sub.2, followed by using Na.sub.2 SO.sub.3 solutions to absorb NO.sub.2.
Japanese Patent No. 77,122,264, which uses ClO.sub.2 to oxidize NO to NO.sub.2, and followed by ammonium solutions to neutralize SO.sub.2 and NO.sub.2.
Japanese Patent No. 77,125,469 uses ClO.sub.2 to oxidize NO to NO.sub.2, followed by scrubbing NO.sub.2 and SO.sub.2 with an alkaline quinone/hydroquinone solution.
West German Patent No. 2,559,546 uses O.sub.3 and ClO.sub.2 to oxidize NO to NO.sub.2, followed by scrubbing NO.sub.2 and SO.sub.2 with an aqueous solution of alkali sulfite containing sulfide, polysulfide, thiosulfate, or thiourea as an oxidation inhibitor.
Japanese Patent No. 78,144,456 uses O.sub.3 to oxidize NO to NO.sub.2, followed by scrubbing with a desulfurization solution to remove NO.sub.2 and SO.sub.2.
Japanese Patent No. 79,33,274 uses CaCl.sub.2 solution to neutralize SO.sub.2 and ClO.sub.2 together with near UV radiation to oxidize NO and NO.sub.2 to HNO.sub.3.
What is needed is a simple process to dissolve NO or its reaction products in aqueous alkaline solution followed by reduction using bisulfite/sulfite with a nitrogen-containing compound or an SH-containing compound which can react with HNO.sub.2 rapidly to produce environmentally acceptable products such as nitrogen. Preferred compounds are urea and sulfamic acid, especially sulfamic acid. The present invention accomplishes this result.